The overall objectives of this project are to examine the mutagenic effects of ionizing radiation in human cells. The research proposed for the coming grant period focuses on the study of mechanisms of radiation induced mutations in human lymphoblastoid cells by use of cellular and molecular probes. The specific aims are: 1) To measure mutations at a series of small marker loci in order to determine whether ionizing radiation produces point mutations; 2) to develop molecular techniques to probe DNA and RNA isolated from radiation-induced mutants at the HGPRT locus to determine if deletions or other rearrangements have occurred; 3) to utilize these molecular techniques to examine a collection of mutants induced by acute x-irradiation, 125IdUrd, and low-dose-rate irradiation from tritiated water; 4) to perform HGPRT enzyme assays on each mutant, and cytogenetic analyses of those mutants which show molecular evidence of rearrangements; 5) to continue and extend cellular studies initiated in the previous gant period which suggest the possibility of a nonlinear response for mutation as a function of time of exposure to low dose rates of ionizing radiation. Such information should enhance our knowledge of the types of DNA modification which are responsible for the important biological effects of ionizing radiation. Furthermore, knowledge of the types of DNA lesions responsible for mutagenesis may offer a scientific basis for predicting the nature of the dose response relationship at low doses and low dose-rates.